In systems for separation of minerals by use of a heavy liquid medium, which are well-known in the art, a mixture of solid particles of different specific gravities, such as crushed ore or coal with waste constituents, is introduced into a heavy liquid medium having a specific gravity between the specific gravities of the various solid particles to be separated. In general, the solid particles having a specific gravity less than that of the heavy liquid float to the surface, while the solid particles having a specific gravity greater than that of the heavy liquid sink.
The liquid medium itself is normally an aqueous suspension of a finely divided solid such as magnetite, ferrosilicon, hematite, galena, borates, silica, or mixtures of one or more of these with other minerals. However, any liquid medium will suffice if it has the necessary specific gravity properties. Some degree of agitation or turbulence is necessary to maintain a substantially uniform suspension of the liquid media. Rotating drums are often used to supply the necessary agitation.
In many such systems the lighter floating particles of the separable minerals are carried away by allowing the liquid medium to overflow from a rotating mixing or separating chamber. The floating particles may then be separated from the liquid by screening or other well-known techniques.
Many such devices also employ blades or scoops to remove the heavier sunken product from the rotating drum.
Despite the current flow created by liquid overflowing from the separator chamber of such separators, the turbulence caused by the rotation of these devices often causes some lighter solids to be caught by blades or scoops. As a result, these lighter solids are carried out with the heavier material. Fixed parallel division walls such as those disclosed in U.S. Pat. No. 1,559,938 do not provide the solids segregation necessary to solve this problem, particularly in more turbulent overflow separators. As a result, a great many of the lighter particles remain dispersed throughout the rejected heavier product. In the case of coal washing separators, where the lighter product is coal, substantial amounts of usable coal are wasted in the heavier reject from these machines due to the inefficiency of the mechanical separating apparatus.
Another wall structure used in the prior art consisted of a pair of elongated L-shaped walls which were mounted in a rotating drum with the longer sides parallel to the axis of rotation. While this structure somewhat reduced the amount of lighter solids which were removed with the heavier sunken product, the L-shaped wall design created dead spaces within the separation chamber where no separation was accomplished, and thereby substantially reduced the area of contact between the solids and the liquid medium.
Also, during shutdown periods such as evenings or weekends, the fine particulate suspension in the liquid medium tends to sink to the bottom surface of the separator drum or chamber. When the apparatus is started, this builtup magnetite or other material requires several minutes to return completely to suspension in the liquid. With the L-shaped wall design discussed above, the walls often catch or bind on the agglomerated mass of particulate so formed, thereby bending the wall or causing other damage to the structure of the separation apparatus. Any fixed wall or other dividing structure mounted in the interior of the drum must be positioned to avoid such binding or catching.
Most raw mineral products also include a substantial number of so-called "middling" particles which may consist of mixtures of light and heavy minerals or pieces of a desired mineral product with heavier impurities on their surface. The presence of such middlings in the purer material lowers the grade of the product, or in a washing operation, increases the amount of impurities in the clean product. However, the inclusion of these particles in the discarded reject fraction results in a substantial economic loss and a higher imperfection rate for the separation system. Several systems have been proposed by which these middlings can be separated from the purer product and then treated in an additional step to further separate the pure or clean product from other minerals and impurities. For example, U.S. Pat. Nos. 2,795,331 and 3,344,918 describe apparatus for separating solid materials into three products of differing specific gravities by heavy media processes.
Previous systems such as those disclosed in these patents have not been able to achieve consistently superior results in terms of efficiency, however, since substantial amounts of lighter particles are not separated from the heavier products. As discussed above, the use of such systems for coal washing results in the loss of significant percentages of usable coal in the reject fraction.
Retention or residence time of the solids in the separating medium also has a critical influence on the quality and completeness of separation. In particular, increased residence time allows the float product more time to rise to the surface of the heavy medium and move into the overflow current area and away from any blades or other means for removing the sink product from the separation apparatus. Thus, any means by which residence time can be increased without adversely affecting other aspects of the system are desired.
The volume of the separation chamber or contact area is also a critical factor in terms of the amount of raw solids which the system will accommodate in a given amount of time. If this separation chamber contact area can be increased, more product can be processed or the speed of the overall operation can be increased.
Accordingly, it is a primary object of this invention to improve apparatus for separating solids by means of a liquid medium.
It is a further object of the invention to recover a substantially more of the lighter product in separation process.
Yet another object of the invention is to increase the residence time of the solids in the separating apparatus and thereby the quality of separation.
A still further object of the invention is to increase the area of contact of the liquid medium and the solids within the separation apparatus and thereby to accommodate a greater volume of solids.
It is a further object of the invention to substantially reduce the migration of lighter solids toward the portion of the separator where heavier solids are evacuated in the separation apparatus.
Another object of the invention is to provide improved apparatus for liquid media separation of solids into three or more products of different specific gravities having the above expressed objects and advantages.